The plurality of capacitors connected in series between a pair of bus lines of a DC circuit of a topology serves to stabilize and smooth a voltage applied between the bus lines which may otherwise be subject to unwanted dynamic changes due to variable input or output currents. In principle, the electric capacitance between the bus lines required for stabilizing and smoothing the voltage applied between the bus lines may be provided by a single capacitor or even by single capacitors connected in parallel between the bus lines. However, such a single capacitor would then be subjected to the full voltage applied between the bus lines. In modem photovoltaic power plants, this voltage typically ranges from 400 to 600 V but may reach or even exceed 1,000 volts. A single capacitor connected between the bus lines must have a working voltage clearly above the maximum voltage applied between the bus lines. Capacitors with a suitable capacitance and a high working voltage, such as on the order of 1,000 volts and higher, are very expensive. For this reason, it is a usual practice to connect two or more low cost capacitors having a lower working voltage in series to provide the necessary capacitance between the bus lines. Each of these capacitors is only subjected to a fraction of the total voltage applied between the bus lines. If one of these capacitors fails, however, the remaining capacitors are additionally subjected to the fraction of the voltage between the bus lines previously covered by the failing capacitor. Particularly with a low overall number of capacitors connected in series, such as for example only two, the remaining capacitors may thus be subjected to a voltage much higher than their working voltage. As a result, they may also break down so that the full voltage between the bus lines is shortcut by the failing and broken down capacitors.
To avoid applying such an excess voltage to the remaining capacitors of a plurality of capacitors connected in series between the bus lines from a failing capacitor of the series, the integrity of the plurality of capacitors has to be surveyed continuously.
US Patent Application Publication 2007/0291522 A1 discloses an interconnection inverter device in which the voltages dropping over each of two capacitors connected in series between two bus lines leading to the inverter are monitored. A controller controls opening or closing of an opening/closing unit connected to either one of the bus lines based on the monitored voltages. If a short-circuit fault occurs in one of the capacitors, a high voltage between the two bus lines is prevented from being applied to the remaining capacitor in that the one bus line in which the opening/closing unit is connected is interrupted towards the DC-power source, i.e., in that one end of the plurality of capacitors is disconnected from the DC-power source.
According to US 2007/0291522 A1, surveying the integrity of the plurality of capacitors requires great effort. One voltage meter is required for each capacitor. Further, the voltage drop over the capacitors have to be monitored carefully and compared to each other or to the total voltage applied between the bus lines.
EP 1 463 188 A2 discloses a circuitry arrangement for limiting the voltage applied between two bus lines by one or a plurality of photovoltaic panels, which is arranged at the input side of an inverter comprising a buffer capacitance arranged between the two bus lines and feeding electric energy from the photovoltaic panel or the plurality of photovoltaic panels into a public power grid. This circuitry arrangement includes a pulsed switch arranged in one of the bus lines, and a voltage meter measuring the voltage between the two bus lines behind the pulsed switch. By pulsing the switch, the effective voltage between the two bus lines behind the switch can be limited to a smaller value than the output voltage of the photovoltaic panel or photovoltaic panels.
There is a need for a simpler but nevertheless effective device for securely surveying the integrity of a plurality of capacitors connected in series between a pair of bus lines arranged to be connected to a DC-power source.